October 26, 2012

Ancient Climate Change Made Hippos Smaller

Have you heard of giant hippopotamuses wandering along the banks of the Elbe in Germany lately? They are not a common sight today, that is for sure, but 1.8 million years ago, hippos were a prominent part of European wildlife along with other megafauna such as woolly mammoths and giant cave bears.

A new study by paleontologists, published in the journal Boreas, claims that the changing climate during the Pleistocene may have forced Europe's hippos to shrink to pygmy size. The climate change eventually forced the hippos to migrate to warmer climates as well.

"Species of hippo ranged across prehistoric Europe, including the giant Hippopotamus antiquus, a huge animal which often weighed up to a ton more than today's African hippos," said Dr Paul Mazza from the University of Florence. "While these giants ranged across Spain, Italy and Germany, ancestors of the modern Hippo, Hippopotamus amphibius, reached as far north as the British Isles."

For 1.4 million years, hippos were a constant feature of Europe during the climatically turbulent Pleistocene, which saw 17 glacial events. The research team investigated the impact and subsequent cost this dramatic climate change would have had.

Mazza and his colleagues focused on fossils found across the European continent - ranging from Germany's Untermassfeld in Thuringia, to Italy's Castel di Guido, Collecurti and Colle Lepre — which were compared to a database of measurements taken from modern African and fossil European hippos.

"The German fossil from Untermassfeld is the largest hippo ever found in Europe, estimated to weigh up to 3.5 tons," Mazza said in a press release. "The Collecurti specimen was also large, but interestingly even though it was close in both time and distance to the Colle Lepre specimen the latter specimen was 25% smaller. A final specimen, an old female from Ortona in central Italy, was smaller again. It was 17% smaller than the Collecurti fossil and approximately 50% lighter."

A clear size threshold separates hippo specimens during different parts of the Pleistocene age. The largest hippos ever known existed during the early Pleistocene, while during the middle Pleistocene smaller specimens emerged.

"We believe the size difference was connected to the changing environmental conditions throughout the Pleistocene," said Mazza. "The Ortona hippo, the smallest of the specimens, lived in a climate where glacial cycles turned colder, while cold steppes replaced warm ones across the Mediterranean."

Temperature drops and rainfall during the Pleistocene changed plant life across Europe, resulting in an expansion of grassy steppes. Hippos might be expected to thrive in this new environment as they were grazers, but they unexpectedly appeared to shrink until the warm periods of the late Pleistocene. The hippos re-gained their former size when the forests and woodlands re-colonized the steppes.

European environments trended towards cooler and drier conditions, which made hippo sizes fluctuate in response. They achieved giant sizes during warmer and more humid stages, while during non-ideal conditions they shrank to small or very small sizes.

"While hippos are normally considered indicators of warm, temperate habitats this research shows that temperature was not only the controlling factor for their ancient ancestors," concluded Mazza. "Our research suggests other factors, such as food availability, were equally important. Appreciating the importance of factors beyond temperature is of great significance as we consider how species may adapt to future ecological and environmental changes."